Many environmental agents, eg. polycylcic aromatic hydrocarbons, have interesting fluorescence properties which can be used to monitor their interaction with biological molecules. The aim of the present work is to improve the utility of available fluorometers. A simple digital circuit has been designed and constructed for use in conjunction with a phase modulation fluorometer. This modification will make it possible to distinguish the intensity contribution from two fluorescent species in a given system provided that their excited states lifetimes differ sufficiently. In addition a simple procedure has been devised to adjust phase modulation fluorometers so that reproducible lifetime measurements can be determined.